| Shape memory alloy (SMA) is a kind of material, in which phase transformation will take place at certain temperature and stress. Due to their particular properties, such as shape memory effect, pseudoelastisity and ferroelasticity, shape memory alloys have been receiving increasing attention in recent years, and are extensively used in many fields, such as industry, aviation, national defense, instruments and medical devices, etc.As a smart material, the study of constitutive relation is critical for its application and development. It has been recognized that thermomechanical properties of a material are determined by its microstructure. It was found experimentally that the pseudoelastic behavior of NiTi shape memory alloys under different loading paths is distinctly different, which can be attributed to the difference of the microstructures during transformation. It is necessary to develop a multi-scale analysis method to study response characteristic and microstructure dependence, and establish a constitutive relation, in which the microstructure characteristic is considered.In this dissertation, the differences of the responses of a NiTi SMA microtube subjected to tension and torsion are systematically investigated. Through special designed device, SEM experiment is carried out to study surface morphology under certain stress path. Finite element analysis is performed to describe mechanical behavior of tensile and torsion of NiTi SMA. The pseudoelastic behavior of a NiTi microtube is analyzed and compared with experimental results. The main work and conclusions in this dissertation are as follows:(1) The progress on the research of the pseudoelastic behavior of shape memory alloys is reviewed. The difference of the macroscopic behavior during tension and torsion and the evolvement of microstructure are analyzed. It is supposed that alternative parallel austenite and martensite fine strips uniformly distributed at the surface of the NiTi SMA microtubes subjected to torsion phase. The research close centers on this suspicion to launch.(2) In view of different phase-transformation microstructure under different loading paths, a special device is designed and processed to load sample. The surface morphologies of the deformed thin-walled NiTi SMA microtube specimens are observed with scanning electron microscope(SEM) and optical microscope, and pseudoelastic experiments are performed by same device. SEM observation shows alternative austenite and martensite striae at the surface of the NiTi SMA microtubes subjected to pure torsion. But for the microtubes subjected to pure tension, macroscopic martensite bands can be observed, which can be attributed to the localization and growth of martensite phase. The test confirmed our suspicion.(3) The mechanical response of a NiTi shape memory alloy under tension and torsion is analyzed with the commercial FE code ABAQUS/standard. The change of the mechanical response, which is the combination of the individual responses of different phases and the interphase constraint, is investigated at different stage. The result shows that interaction between phases will change remarkably with the change of volume fraction, the smaller of volume fraction of soft phase means the larger constraining force between interphase. The difference of the macroscopic behavior during tension and torsion are analyzed with different microstructure characteristic.(4) Based on test and finite element analysis, assumpe that a NiTi SMA is an aggregate of numerous cells, and each cell is composed of many alternative fine lamellas of austenite and martensite. First, constitutive equations of austenite and martensite are derived through simple mechanical model. The effect of the localization and the growth of the martensite phase are taken into account by introducing an additional local volumetric strain, which relaxes the over-constraint induced by the identical in-plane strain assumption that is accurate as the interlamellar spacing is infinitesimal. Then, the equation of representative volume element is derived with simplification supposition. Using Hill self-consistent scheme and Eshelby tensor, The elastic-plastics equation of NiTi SMA is established. The multi-scale constitutive system is get.(5) Take use of multi-scale constitutive description,the corresponding numerical algorithm is developed. The pseudoelastic behavior of a NiTi SMA subjected to pure tension and pure torsion is analyzed. It shows that the developed constitutive model can well describe the typical"stress drop"of the materials subjected tensile deformation, and the difference between the pseudoelastic responses corresponding respectively to tension and torsion. |
